1. Technical Field
The present invention relates to a transport roller, a transport unit, and a printing apparatus.
2. Related Art
In the past, various printers have been provided as printing apparatuses. These printers are configured such that recording media such as paper are transported to a printing unit by a transport roller (paper feeding roller) and a driven roller, and after printing is performed, the media are discharged by the discharge roller (driving roller) and the driven roller (GIZA roller).
In such a printer, the transport roller rotates in a state where the paper is interposed between the transport roller and the driven roller, so that the paper is moved in a sub scanning direction perpendicular to a moving direction of a carriage. Since the paper must be transported to the recording position with high accuracy and be sequentially fed in synchronization with the printing speed, a high transportation force is required.
In order that the transport roller has a high frictional force, Japanese Patent No. 3271048 discloses a technique in that a large number of protrusions are formed on a circumferential surface of a metallic round bar by an embossing work.
In the technique, there is a problem that workability is very poor at forming the protrusions on the shaft-like (cylindrical) surface along the circumferential direction. In addition, since a solid material is used, the cost is also increased.
In view of the above situation, JP-A-2006-289496 discloses with the aim of cost reduction, in that a metal plate is subjected to a bending work to form a shaft (cylindrical shaft) in a cylindrical shape (hollow shape) and the cylindrical shaft is used instead of the solid metallic round bar member.
However, when the cylindrical shaft as disclosed in JP-A-2006-289496 is used as the transport roller in order to achieve cost reduction and weight reduction, it is difficult to make the cylindrical shaft have a high frictional force.
For example, it may be considered that the surface is beaten by a pressing work to form protrusions as in the case disclosed in Japanese Patent No. 3271048. However, in this case, since the shaft is hollow, it is easily deformed. In addition, a joining portion which is formed when the bending work is carried out is open.
As described above, when the joining portion is open, a minute groove caused by the joining portion is formed in the transport roller to be obtained. When the paper (recording medium) comes into contact with the groove at the time of feeding the paper, the contact resistance differs from that when the paper comes into contact with other surfaces. As a result, the transport speed is changed which acts as one factor causing uneven transportation. In addition, a so-called ink jet printer, which forms images by ejecting fine droplets, necessitates very high accuracy in feeding paper. Therefore, in particular, the occurrence of uneven transportation caused by the groove becomes problematic.